Apparatuses are known which are used to retain curved sheets in position. Such devices comprise a plurality of devices spaced apart at regular intervals on a support plane in a grid-like arrangement. Each device is provided with means for adhesion to the sheet to be retained in order to hold the sheet in position. Furthermore, each of the devices is positioned automatically, independently of all other devices, at a predetermined height so as to match the profile of the sheet in the region of the support device. The sheet is thus supported at a certain number of points on its surface, regardless of the configuration thereof.
In industry, particularly the aircraft industry, it is frequently necessary to perform precision machining on sheets or similar two dimensional units having a curved surface. Obviously, for precise machining to be achieved, it is essential to position the sheet with maximum accuracy and to retain it in a fixed position whilst the various machining operations are being effected. The difficulties which arise when the sheet or panel has length and breadth dimensions of the order of meters but a thickness of the order of millimeters and has a curved surface, but which must, nevertheless, be retained in position without being deformed or damaged will be readily apparent.
The use of fixed profiles corresponding to the geometry of a particular panel would be uneconomic in view of the relatively high number of profiles which it would be necessary to provide to ensure that any panel could be retained.
For this reason, apparatuses have been developed which consist of a plurality of support members which can be positioned so as to produce a support structure corresponding to the profile of the sheet. The sheet is then attached to the support members. One particular apparatus of this type is disclosed in U.S. pat. No. 4,491,306. Reference should now be made to FIG. 1 of the accompanying drawings which shows, schematically, an apparatus as disclosed in U.S. Pat. No. 4,491,306. Such an apparatus comprises a support member A which can be positioned at various heights. The upper portion of the support member A is provided with a hemispherical cup B within which a cap C of a substantially complementary shape is rotatable.
The cap C is provided, on its upper edge, with a suction cup D and the base thereof is connected to a spring E which is mounted on the base of the cup B and retains the suction cup D in the position illustrated in the figure.
Within the cup C, a pin P is provided, which pin extends radially. The upper end of the pin P coincides with the center of rotation of cap C, as is disclosed in column 4 at lines 54-57 of U.S. Pat. No. 4,491,306.
The rotatability of cap C within the cup B allows the suction cup D to correspond to the inclination of a sheet which is rested thereon. The sheet is retained in position by depressurising the interior of the cups D. The pin P also provides a further point of support for the sheet.
The possibility of automatically controlling the raising of support A so as to bring pin p in contact with the sheet being machined is also foreseen in the above-mentioned patent. To achieve this, provision is made, as described in column 5 at lines 58 et seq, for the mounting of a proximity sensor.
Such an arrangement does not, however, fully satisfy industrial requirements. The fact that the sheet itself is used as a contact for positioning the supports poses considerable problems, in that it is necessary to provide means whereby a panel having length and breadth dimensions of the order of meters can be positioned with maximum accuracy so that the support members may then be adjusted. A further disadvantage resides in the fact that the position of the upper extremity of the pin P, which constitutes a support point for the sheet, is fixed with respect to the edge of the suction cup D. In consequence, the device is not automatically adjustable so as to retain sheets of different curvature. To achieve this, the pin must be replaced by another pin of an appropriate length. Thus, in the case of concave sheets it is necessary to use a pin which projects outwardly from the suction cup and, in the case of a convex sheet, a pin which does extend as far as the suction cup.
If an excessively long, that is to say, projecting pin is used, the sheet does not adhere adequately to the suction cup, so that adhesion to the corresponding support is thus correspondingly inadequate. A pin which does not extent as far as the suction cup provides good adhesion, but it does not provide the central support point for the sheet.